This invention relates to thermal energy systems employing heat exchangers. In particular, the invention relates to a thermal energy system with passive back-flushing of a heat exchanger, and methods for passively back flushing systems.
Thermal energy systems incorporating heat exchangers typically comprise a primary loop, from which heat is supplied or removed, and a secondary loop, to or from which heat is transferred. The heat exchanger transfers heat between the primary and the secondary loop. A heat transfer fluid is circulated through the primary loop, supplying heat to, or removing heat from, the primary side of the heat exchanger. A secondary fluid to which heat is supplied or from which heat is removed flows through the secondary side of the heat exchanger. The primary and secondary sides of the heat exchanger typically have numerous small passageways in close association through which the fluids flow, which facilitate the transfer of thermal energy therebetween.
Modern heat exchangers are compact and offer high performance, i.e., high rates of heat transfer. High performance is usually achieved by making the passageways very small, and providing many of them. However, as the size of the passageways is reduced, they become more prone to fouling or complete blockage due to the accumulation of sediments, scale, and mineral deposits that may be present in the circulating fluid. Fouling of the heat exchanger leads to a substantial drop in performance of the system. Specific measures taken to minimize fouling include monitoring and control of the chemical composition of the fluids, frequent disassembly for cleaning of the flow passages, and oversizing of heat transfer surfaces and flow passages to ensure that they will have sufficient capacity even when operating at decreased effectiveness due to fouling. In the case of thermal systems for heating potable or process water, there is a high probability that mineral salts and other impurities may be present in the water. In such cases a potential for fouling of the heat exchanger exists if the exchanger is not routinely cleaned or flushed of accumulated matter. In many applications, such as residential and small commercial installations, monitoring of the chemical composition of the water, routine disassembly and cleaning of the heat exchanger, or oversizing are not practical due to the associated costs.
According to one aspect of the invention there is provided a thermal energy system, comprising: a heat exchanger for transferring thermal energy between a source and a load, the heat exchanger having a primary side associated with said source, and a secondary side for conducting a fluid associated with said load; wherein the secondary side of the heat exchanger is passively back-flushed upon consumption of a portion of said fluid. In certain embodiments, the thermal energy system further comprises a storage tank associated with the load.
In one embodiment, the load is a hot water supply and the fluid is water. In another embodiment, the load is a chilled water supply and the fluid is water.
In another embodiment, a thermal energy system of the invention further comprises a back-flushing valve, wherein the back-flushing valve passively controls back-flushing of the secondary side of the heat exchanger. In certain embodiments, the back-flushing valve is activated by at least one of flow rate, temperature, and pressure of the fluid. In a preferred embodiment, the back-flushing valve is activated by flow rate of the fluid. In further embodiments, the back-flushing valve provides a bypass flow when the valve is closed. In some embodiments the bypass flow is about 1% to about 20% of a flow rate during consumption of the fluid.
In certain embodiments, the source is a heat source selected from solar heat, waste heat, geothermal heat, industrial process heat, a heat pump, a boiler, and a furnace. In a preferred embodiment, the heat source is solar heat.
In a further embodiment of the invention there is provided a thermal energy system comprising: a heat exchanger for transferring thermal energy between a source and a load, the heat exchanger having a primary side associated with said source, and a secondary side for receiving fluid to be heated or cooled and outputting said heated or cooled fluid, the fluid flowing through the secondary side of the heat exchanger in a first direction; an input for receiving mains fluid; and a back-flushing valve for controlling flow of the heated or cooled fluid and the mains fluid; wherein, upon consumption of a portion of the heated or cooled fluid, the back-flushing valve passively directs mains fluid through the secondary side of the heat exchanger in a second direction opposite to that travelled by the heated or cooled fluid. In one embodiment, the back-flushing valve provides a bypass flow when the valve is closed. In some embodiments the bypass flow is about 1% to about 20% of a flow rate during consumption of the fluid.
According to a further aspect of the invention there is provided a module for a thermal energy system including a storage tank associated with a load, said module comprising: a heat exchanger for transferring heat from a heat source to a load, the heat exchanger having a primary side for receiving heat from a heat source and a secondary side for receiving water to be heated and outputting said heated water to the load, the heated water flowing through the secondary side of the heat exchanger in a first direction; an input for receiving mains water; and a back-flushing valve for controlling flow of the water to be heated and the mains water; wherein, upon consumption of a portion of the water to be heated, the back-flushing valve passively directs mains water through the secondary side of the heat exchanger in a second direction opposite to that travelled by the water to be heated. In one embodiment, the back-flushing valve provides a bypass flow when the valve is closed. In some embodiments the bypass flow is about 1% to about 20% of a flow rate during consumption of the fluid.
By another aspect of the invention there is provided a method for passively back flushing a heat exchanger in a thermal energy system, comprising: providing a heat exchanger for transferring thermal energy between a source and a load, the heat exchanger having a primary side associated with said source, and a secondary side for conducting a fluid associated with said load; providing a source of excess fluid; flowing the fluid through the secondary side of the heat exchanger in a first direction; and upon consumption of at least a portion of the fluid by the load, passively flowing said excess fluid through the secondary side of the heat exchanger in a second direction opposite to the first direction.
In one embodiment of the method, the thermal energy system is a hot water system. In certain embodiments, the heat source is selected from solar heat, waste heat, geothermal heat, industrial process heat, a heat pump, a boiler, and a furnace. In a preferred embodiment, the heat source is solar heat. In yet another embodiment of the method, the thermal energy system is a chilled water system.
In one embodiment, the back-flushing step is activated by at least one of flow rate, temperature, and pressure of the fluid. In a preferred embodiment, the back-flushing step is activated by flow rate of the fluid.